CA2559562C - Steel for mechanical parts, method for producing mechanical parts from said steel and the thus obtainable mechanical parts - Google Patents

Abstract

Steel for mechanical parts, characterized in that its composition is, in percentages by weight: - 0.19% <= C <= 0.25%; - 1.1% <= Mn <= 1.5%; - 0.8% <= If <= 1.2%; 0.01% <= S <= 0.09%; - traces <= P <= 0.025%; - traces <= Ni <= 0.25%; -1% <= Cr <= 1.4%; - 0.10% <= Mo <= 0.25%; - traces <= Cu <= 0.30%; 0.01% <= Al <= 0.045%; - 0.010% <= Nb <= 0.045%; 0.0130% <= N <= 0.0300%; - optionally traces <= Bi <= 0.10% and / or traces <= Pb <= 0.12% and / or traces <= Te <= 0.015% and / or traces <= Se <= 0.030% and / or traces <= Ca <= 0.0050%; the remainder being iron and impurities resulting from the preparation, the chemical composition being adjusted so that the average values J3m, J11m, J15met J25m of five Jominy tests are such that: .alpha. = | J11m - J3m x 14/22 - J25mx 8/22 | <= 2.5 HRC; and ~ = J3m - J15m <= 9 HRC. Process for manufacturing a mechanical part using this steel, and mechanical part thus obtained.

Description

Steel for mechanical parts, parts manufacturing process mechanics using it and mechanical parts thus produced The invention relates to the field of iron and steel, and more particularly steels for mechanical parts such as sprockets.
Knife steels must have a high resistance to contact fatigue. Most of the time, the parts machined from these steels undergo a carburizing or carbonitriding treatment, aimed at provide them with superficial hardness and mechanical strength sufficient, while retaining them good tenacity at heart thanks, in particular, to a carbon content of the order of 0.10 to 0.30% only. The carbon content the cemented layer can be up to about 1%.
Various documents describe gilding steels intended to be casehardened. US-A-5,518,685, in which the contents of Si and Mn are kept within relatively low limits (0.45 to 1% and 0.40 to 0.70% respectively) to avoid intergranular oxidation during cementation. JP-A-4-21757 discloses steels for gearing intended to be cemented by plasma or under reduced pressure, then shot blasted, which may have higher Si and Mn contents than the previous ones. They have a high resistance to the surface pressure experienced by the pinion, the duration of which life is so high.
WO-A-03 012 156 proposes a steel for mechanical parts, such as than pine nuts, whose composition is: 0.12% 5 C 5 0.30%; 0.8% 5 If 5 1.5%; 1.0% 5 Mn 5 1.6%; 0.4% Cr 5 1.6%; Mo, 0.30%; Neither 0.6%; Al 5 0.06%; Cu 0.30%; S, 0.10%; P 0.03%; Nb 5 0.050%. This steel presents the advantage of minimizing the plastic deformations in service of the whole of the piece, thanks, in particular, to a judicious balancing of the contents in silicon and manganese. Preferably, carburizing or carbonitriding must take place under non-oxidising conditions, eg under pressure reduced, so that the relatively high levels of silicon and manganese do not do not lead to intergranular oxidation problems.
Usually carburizing or carbonitriding takes place at a temperature of the order of 850 to 930 C. However, the current trend is to seek to perform this operation at higher temperatures (cementation or carbonitriding at high temperature), of the order of 950 to

2 1050 C. This increase in the treatment temperature allows either reduce the duration of treatment, at equal cemented depth, or increase cemented depth, with equal treatment time. At the choice of the producer, it is thus possible to increase the productivity of the installation, or to increase the performance of the products obtained.
However, the application of a carburizing or high carbonitriding known steel temperatures that have been described poses several problems.
In the first place, the high temperature can lead to a growth of poorly controlled grains, harmful to the mechanical properties of the room.
On the other hand, cementation or carbonitriding is followed by quenching.
during which the piece undergoes deformations. These can require a machining recovery of the part, even in the most serious cases, train it's scrapped. These problems are accentuated when the quenching takes place on a piece having undergone a carburizing or high carbonitriding temperature and not at a more usual temperature.
The object of the invention is to propose to metallurgists practicing the carburizing or high-temperature carbonitriding of mechanical parts, in particular gears, a steel answering the problems previously cited while maintaining the required mechanical properties, and also compatible with carburizing and carbonitriding operations performed at more usual temperatures.
For this purpose, the subject of the invention is a steel for mechanical parts, characterized in that its composition is, in percentages by weight:
- 0.19% 5 C 5 0.25%;
- 1.1% 5 Mn 5. 1.5%;
- 0.8% 5. If 5. 1.2%;
-0.01% 5 S .5 0.09%;
traces 5. P 5 0.025%;
- traces .5 Ni 5. 0.25%;
-1% 5 Cr 5 1.4%;
- 0.10% 5 Mo 5. 0.25%;
-traces 5 Cu 5 0.30%;

3 0.010% 5A (0.045%;
- 0.010% 5 Nb 5. 0.045%;
- 0.0130% 5 N 5. 0.0300%;
Ti 0.005%;
the rest being iron and impurities resulting from the elaboration, the composition chemical being adjusted so that the average values J3m, Jiimi jism and J25m of five Jominy essays are such that:
01J11mJ3mX14I22J25mX8I22I52,5 HRC and = J3n, ¨ J15nõ 5. 9 HRC.
Preferably, the steel is characterized in that its composition is, in weight percent: traces 5 Bi 5 0.10% and / or traces 5 Pb 5 0.12% and / or footsteps 0.015% and 0.030% and / or traces of 0.030% and / or traces of 0.0050%.
Preferably, its composition is adjusted so that J3 ,, ¨ J15 ,, 5. 8 HRC.
Preferably, its composition is:
- 0.19% 5 C 0.25%;
1.2% Mn 5 1.5%;
- 0.85% .5 If _5 1.2%;
- 0.01% FS 0, 0 9%;
traces 5. P 5 0.025%;
0.08% 5 Ni 5 0.25%;
- 1.1% 5. Cr 5 1.4%;
- 0.10% 5 Mo 5 0.25%;
- 0.06% .5 Cu _5 0.30%;
0.010% Al 5 0.045%;
0.015% _5 Nb 5 0.045%;
0.0130% 5 N 5 0.0300%;
Ti 0.005%;
optionally 5 Bi 5 0.07% traces and / or traces <Pb <0.12% and / or traces 5. Te 5. 0.010% and / or traces 5 Se 0.020% and / or traces 5 Ca 5 0.045%, the rest being iron and the impurities resulting from the elaboration.

4 Optimally, its composition is:
- 0.20% C 5 0.25%;
- 1.21% 5. Mn 5. 1.45%;
- 0.85% 5 If 5_ 1.10%;
0.01% 5 S 0.08%;
traces 0.05%;
- 0.08% 5 Ni 5 0.20%;
- 1.10% 5 Cr 5. 1.40%;
0.11% 5 Mo 5 0.25%;
0.08% Cu 5 0.30%;
- 0.010% _5 AI _5 0.035%;
- 0.025% .5 Nb 5 0.040%;
- 0.0130% .5 N 5 .. 0.0220%;
Ti 0.005%;
optionally traces 5 Bi 5 0.07% and / or traces _5 Pb 0.12% and / or traces 0.010% and / or traces 0.020% and / or traces 5 Ca 0.045%, the rest being iron and the impurities resulting from the elaboration.
The subject of the invention is also a method of manufacturing a mechanical part in case-hardened or carbonitrided steel, characterized in that uses for this purpose a steel of the above type on which a machining, carburizing or carbonitriding then quenching.
Preferably, said carburizing or carbonitriding takes place at a temperature of 950 to 1050 C.
The subject of the invention is also a mechanical steel part, such as that a piece of gear, characterized in that it is obtained by the previous process.
As will be understood, the invention is based on a precise adjustment ranges of grades of the main alloying elements, as well as the simultaneous presence, in well-defined levels, of aluminum, niobium and nitrogen.
The desired effects are essentially twofold.

4a First, the choice of grades in the main elements of alloy aims to obtain a Jominy curve without inflection point significantly marked. This condition makes it possible to obtain deformations minimum during quenching. From this point of view, cementation or carbonitriding performed at high temperature is, as has been said, particularly demanding.
It is recalled that the Jominy curve of a steel, which is obtained by means of a standard and standardized test characterizes the hardenability of steel.
She is obtained by measuring the hardness of a cylindrical specimen, quenched by a jet of water watering one of its ends, along one of its generators. The hardness is measured at several distances x (in mm) from the watered end, and the corresponding value is denoted by J. We call Jx ,, the mean value obtained during five tests of hardness at distance X.
As set forth in EP-A-0 890 653 to which the reader is invited to refer for further details, the plaintiff had

5 showed that a composition of the steel providing a curve Jominy without point Inflection was favorable for obtaining very small deformations at Classes quenching after carburizing or carbonitriding. This curve Jominy without inflection point is obtained when the values J11m, J3m, J25m and J15rn satisfy the following relations:
- a = 1Jiim ¨J3m x 14/22 ¨ J25m x 8/2215 2.5 HRC;
## EQU1 ## or, more preferably, 5 8 HRC.
The composition of the steel according to the present invention is therefore adjusted so that this relation is also obtained in his case.
The composition is also adjusted, notably thanks to the presence aluminum, niobium and nitrogen in defined levels, so that the grain size remains controlled, even when carburizing or Carbonitriding takes place at high temperature.
Finally, of course, the composition of steel must provide the mechanical properties sought for the use of the room. From criteria to be monitored more particularly we can mention the depth casehardened (classically defined by the depth at which the measured hardness is 550 HV), the hardness difference between the surface and the core of the cemented part who must be as low as possible to minimize quenching deformations, and the hardness at heart that must be high for the piece to have a good answer to the constraints in service, and thus a good resistance in endurance and in tired.
The invention will be better understood on reading the description which follows, given with reference to the attached figure, which shows the Jominy curves of four reference steels and three steels according to the invention.
The steel according to the invention is intended primarily for the manufacture of highly stressed mechanical parts such as elements of gears, intended to be case hardened or carbonitrided (preferably low pressure or non-oxidizing atmosphere to avoid oxidation the most oxidizable elements), both at usual temperatures of

6 850-930 C approximately at high temperatures of the order of 950-1050 C.
These parts must have a high endurance in fatigue, a good toughness and only weakly deformed during heat treatments such as quenching after carburizing or carbonitriding. He has the following composition (all percentages are percentages weight).
Its carbon content is between 0.19 and 0.25%. these grades are usual for steels of pignonnerie. On the other hand, this beach authorized an adjustment of the contents of the other elements which makes it possible to obtain the form desired for the Jominy curve. The minimum content of 0.19% is, moreover, justified by the hardness after quenching it allows to obtain. Beyond of 0.25%, the hardness may be too high to retain steel machinability desirable. The preferred range is 0.20-0.25%.
Its manganese content is between 1.1 and 1.5%. The value minimum is justified by obtaining the desired Jominy curve in conjunction with the contents of the other elements. Beyond 1.5% there is the risk of segregation, and also of band structures during annealing. Moreover, such a high content would provoke the development an excessive attack of the refractory lining of the steelmaking ladle. He ... not It would be undesirable to further tighten this range of grades because obtaining the desired precise grade at the steel mill could be exaggerated difficult. The preferred range is 1.2-1.5%, better 1.21-1.45%.
Its silicon content is between 0.8 and 1.2%. In this range, the desired shape of the neck Jominy can be obtained in conjunction with the contents of the other elements. The minimum value of 0.8% is justified by obtaining the desired hardness of heart, as well as by the limitation of gap hardness between surface and heart after carburizing or carbonitriding. At-of the than 1.2%, there is a risk of excessive segregation silicon, if it segregates itself little, tends to accentuate the segregation of others elements. There would also be an increased risk of oxidation during the carburizing or carbonitriding. The preferred range is 0.85-1.20%, better 0.85-1.10%.
Its sulfur content is between 0.01 and 0.09%. the value minimum is justified for obtaining correct machinability. Beyond

7 0.09% there is a risk of too sensitive reduction of hot forgeability.
The Preferred range is 0.01-0.08%.
Its phosphorus content is between traces and 0.025%. Of Generally speaking, the standards in force tend to require maximum phosphorus of this order. Moreover, beyond this value, there is a risk of synergy with niobium causing embrittlement of steel then hot forming and / or continuous casting of steel in the form of blooms or billets. Preferably, the phosphorus content is at most 0.020%.
io Sa nickel content is between traces and 0.25%. This element, voluntarily introduced at higher levels, would increase unnecessarily the cost of the metal. In practice, we can be satisfied with the nickel content naturally resulting from the melting of raw materials of casting, without voluntary addition. The preferred range is 0.08-0.20%.
Its chromium content is between 1.00 and 1.40%. In this range, in conjunction with the contents of the other elements, one can obtain the shape of the desired Jominy curve. In addition, the minimum content of 1.00%
= allows to obtain a good hardness to heart_ Beyond 1.40%, one increase unnecessarily the cost of elaboration. The preferred range is 1.10-1.40%.
Its molybdenum content is between 0.10 and 0.25%. In this range, in conjunction with the contents of the other elements, we obtain the shape of the Jominy curve and desired heart hardness. Range preferential is 0.11-0.25%.
Its copper content is between traces and 0.30%. Here again, as for nickel, we will generally keep purely and simply content obtained after melting raw materials. Beyond 0.30%, we Degrade ductility and tenacity at the heart of the room. Range preferential is 0.06-0.30%, better 0.03-0.30%, so as to optimize the Jominy curve shape and hardness after quenching.
Its contents of aluminum, niobium and nitrogen must be controlled within precise limits. Indeed, they are elements that, in interaction, provide control of the fineness of the grain of the metal. This finesse is desirable for obtaining good toughness in the cemented layer or carbonitride, good fatigue performance and a reduction in dispersion

8 deformation during quenching. Moreover, it also has its importance in obtaining the desired shape of the Jominy curve. Size control of grain is, in the context of the invention, even more important than steel must be able to undergo carburizing or carbonitriding at high temperatures without excessive growth in grain size.
This control of the grain is essentially by the precipitation of nitrides and carbonitrides of aluminum and / or niobium. To obtain it, he should therefore a significant presence of these two elements, as well as nitrogen to a considerably higher than that usually obtained at the Following an elaboration carried out under normal conditions.
The aluminum content must be between 0.010 and 0.045%.
In addition to the grain control function already mentioned, this element deoxidation of steel and its cleanliness in terms of inclusions of oxides. In-below 0.010%, its effects, from these latter points of view, would be insufficient. Above 0.045%, the cleanliness in risk inclusions of oxides to be insufficient for the intended applications. Range preferential is 0.010-0.035%.
The niobium content must be between 0.010 and 0.045%. In-below 0.010% the control effect of the grain would not be sufficient, in especially for the lowest aluminum contents. Above 0.045%, it there is a risk of cracks appearing during the continuous casting of the steel, especially if a synergy with phosphorus can occur, as we have reported above. The preferred range is 0.0 15-0.045%, better 0.015-0, 040%.
In conjunction with the aluminum and niobium contents as mentioned, the nitrogen content must be between 0.0130 and 0.0300%
(130 to 300 ppm), so that the adjustment of the grain size and shape of the Jominy curve desired are obtained. The preferred range is 0.0130-0.0220%.
If this appears desirable, one or more of the elements conventionally known to improve its machinability: lead, tellurium, selenium, calcium, bismuth in particular. Their maximum levels are 0.10%, better 0.07%, for Bi, 0.12% for Pb, 0.015%, better 0.010%, for Te, 0.030%, better 0.020%, for Se and 0.0050%, better 0.0045%, for Ca.

9 The other elements are those usually present in rader as impurities resulting from the elaboration, and are not added voluntarily. In particular, it must be ensured that the titanium content born not exceed 0.005%. Indeed, as the steel according to the invention is very rich in nitrogen, beyond this level there is a risk of nitrides and / or coarse titanium carbonitrides, visible by micrograph, which reduce fatigue strength and impair machinability. In addition, titanium would capture nitrogen that would no longer be available for control of the grain.
The invention will now be illustrated by means of examples. The figure to annexed shows the Jorniny curves of four steels whose compositions are given in Table 1. Aces, B, C and D are steels of reference. Steels E, F and G are they, according to the invention.
Steel_ C% Mn% syx, S% P% Ni% CI% Mo% Cu% M% _ 17%, Nb% N%
A 0.236 0.888 0.224 0.015 0.011 0.011 1.194 0.014 0.010 0.021 trace traces 0.0124 (Ref.) B 0.196 1.188 0.069 0.023 0.012 0.208 1.228 0.098 0.1e2 0.021 traces 0.030 0.0179 ref.
C 0.192 1.205 0.845 0.029 0.014 0.080 0.995 0.099 0.1 / 0 0.025 hassle 0.011 0.0110 (ref.) =
D 5.245 1.215 0.840 0.035 0.012 0.086 0.980 0.103 0.098 0.035 traces 0.012 0.0090 =
Jr) E 0.230 1.287 0.920 0.018 0.017 0.201 1.289 0.200 0.211 0.032 traces 0.025 0.0174 F 0.201 1.453 1.191 0.041 0.014 0.139 1.381 0.248 0.122 0.031 = 0.002 0.038 0.0243 (Inv.) G 0.241 1.254 0.852 0.015 0.010 0.189 1.121 0.111 0.109 0.012 traces 0.015 0.0141 Jinv.) Table 1: Sample Compositions In the case of sample A, size a as defined above is equal to 8.7, and the magnitude g as defined above is equal to 19.1.
They are therefore well above the maximum required by the invention. Of made, we see that the Jominy curve presents a very marked point of inflection.
=
TOTAL P.03 In the case of sample B, a is equal to 2.38 and f3 is equal to 11.1.
[3 is therefore not in accordance with the requirements of the invention, and the Jominy curve also presents a significant inflection point, although it is contains niobium and nitrogen within the prescribed limits. The essential reason is 5 that its silicon content is insufficient.
In the case of sample C, a is equal to 3.38 and f3 is equal to 10.7.
Neither a, nor f3 are within the prescribed limits, and the Jominy curve presents a marked inflection point. Cr and Mo are just deschanted values minimum requirements, and especially the nitrogen content is insufficient.
io In the case of sample D, a is equal to 2.845 and f3 is equal to 9.5, which again is outside the prescribed limits. The Jominy curve has a marked inflection point, due to Cr and nitrogen contents insufficient.
On the other hand, for the sample E according to the invention a is equal to 0.41 and [3 is 2.7. The requirements are met and we see that the Jominy curve is almost rectilinear and devoid of point of inflection.
Similarly, for the sample F according to the invention, a is equal to 0.23 and f 3 is equal to 3.7. Again, its Jominy curve is almost straight and devoid of inflection point.
Similarly, for the sample G according to the invention, a is equal to 0.83 and [3 is equal to 6.6. Its Jominy curve is almost rectilinear and devoid of marked inflection point.
We have also studied the cementing behavior of A steels, B and E of Table 1 under normal and high temperature conditions.
temperature.
Cementations at usual temperature (930 C) were carried out under low pressure in similar conditions on samples cylindrical to give the cemented surface a carbon content of 0.75%. These cementation was followed by quenching in a gaseous medium (in this case in nitrogen, but a nitrogen-hydrogen mixture with 10% hydrogen would, for example, could be used) under two different pressure conditions:

bars and 20 bars. It was thus intended to obtain a superficial hardness of 700 to HV and a cemented depth (ie the depth at which the hardness is of 550 HV) of 0.50 mm. The results are given in Table 2 (tests at bars) and in Table 3 (tests at 20 bar).
Steel Hardness HV in Cemented Depth Hardness HV at heart out of area cemented surface (mm) A 760 0.35 263 (Ref.) B 760 0.50 408 (Ref.) E 780 0.48 426 (Inv.) Table 2: Cementation behavior in the case of quenching 5 in gaseous media at 5 bar Steel HV hardness in case hardness HV hardness outside the zone cemented surface (mm) A 780 0.45 318 (Ref.) B 720 0.55 423 (Ref.) C 738 0.53 408 (Ref.) E 750 0.55 524 (Inv.) Table 3: Cementation behavior in the case of quenching in gaseous media at 20 bar These tests show that the reference steel A does not allow to easily reach the cemented depth sought. This is due to his lack of hardenability.
The reference steels B and C and the steel according to the invention E permeate all three to obtain the desired case-hardened depth under conditions of usual carburizing temperature.

The AHV difference between the surface hardness and the hardness at heart is very comparable, for a quenching medium at 5 bar, in the case of reference B and steel according to the invention E (AHV = respectively 352 and 354) and much lower than the reference steel A (AHV = 497). For a quenching medium at 20 bar, on the other hand, AHV is significantly less favorable for the reference steels B and C as for the steel of the invention E (AHV =
respectively 297, 330 and 226). As a result, the residual stresses generated by these differences of hardness, which are at the origin of the deformations during of quenching under severe conditions on cemented parts, can be minimized by the use of steels according to the invention.
Finally, the highest heart hardness is obtained with steel E
according to the invention. So in the case of heavily pinion parts requested in service for which characteristics are sought high mechanical properties (especially high hardness under hardened and at heart), superior to the stresses to which the piece is subjected in service, so as to ensure good fatigue endurance in service, the steel according to the invention is the one which, for carburizing data, will lend itself best to high fatigue endurance in use.
We also carried out high level cementation tests temperature (980 C) on cylindrical samples of steels A and D of reference and E according to the invention described above. Here again the surface cemented had a carbon content of 0.75%. In both cases, we were aiming a superficial hardness of 700 to 800 HV and a cemented depth, hardness of 550 HV, 0.50 mm. Quenching in a gaseous medium (nitrogen) which followed the cementation was carried out under a pressure of 20 bar for steels A and D and only 1.5 bar for steel E. The results are shown in the table 4. It also presents grain size assessments according to the standard ASTM.
Steel Hardness HV Depth Hardness HV to Grain Size Size grain in case hardened surface off ASTM zone in ASTM off (mm) cemented case-hardened layer case-hardened layer A 740 0.50 312 7/9 8/9 (Ref.) D 735 0.59 461 7/8 8/9 (Ref.) 740 0.70 500 8/9 9/10 (Inv.) Table 4: Cementation behavior in the case of quenching in gaseous media at 20 bar (steels A and C) and 1.5 bar (steel E) As in the case of carburizing at the usual temperature of 930 C, the two steels achieve the desired surface hardness.
The invention makes it possible to obtain a cemented depth substantially more important than in the case of reference A, although it has been soaked in much more severe conditions that are known to increase the case-hardened depth all things being equal by elsewhere.
The hardness difference between surface and core is significantly lower in the case of the invention only in the case of references A and D (LHV =
respectively 240 for E, 428 for A and 274 for D). The benefits mentioned highest in deformation during quenching after carburizing at usual temperatures are also found here, even more accentuated.
The hardness at heart is higher in the case of the invention than in the case of reference, despite a pressure of the middle of quenching much more low. The consequences on improving fatigue endurance in mentioned above for tempering at usual temperatures are found also here.
Finally, both in the cemented zone and outside the cemented zone, the steel according to the invention has a grain size ASTM finer than steels of A and D. As a result, it is less sensitive to the risks of magnification grain when carburizing at high temperatures. This is a very good advantage significant because the enlargement of the grain on cemented parts has an effect extremely detrimental to the fatigue resistance of the toes and the tenacity cemented parts. The steels according to the invention are therefore perfectly suitable for use in making gimbal parts (or all other parts for which comparable characteristics are required) hardened or carbonitrided at high temperatures, with all the advantages cost savings, without sacrificing performance said pieces.
We have also carried out other tests of carburizing under low pressure on the reference steel A and the steel E according to the invention.
For low pressure carburizing carried out at 930 C on steel A followed by quenching gas at 20 bar, it takes 72 minutes of carburizing to obtain the targeted carburizing depth of 0.50mm for HV = 550.
With the steel E according to the invention, in low pressure carburizing at 930 C
followed by quenching gas (same gas as for steel A) under 1.5 bar, 30 minutes of cementation are sufficient to obtain the same carburized depth of 0.50mm for HV = 550.
For carburizing under low pressure at high temperature 980 C made on the reference steel A, it takes 30 minutes of cementation and a gas quench at 20 bar to obtain the desired degree of carburizing 0.50mm for HV = 550. 20 minutes of carburizing time under low pressure at 980 C are sufficient to achieve the same depth of carburizing 0.5 mm for HV = 550 for steel E according to the invention and this with a quenching gas under a pressure of only 1.5 bar. The quenching gas used for steels A and E is of course the same.
This shows that the steel E according to the invention makes it possible to reduce carburizing time both at normal carburizing temperature (930 C) than at high temperature (980 C), which reduces costs carburizing (amount of carburizing gas, cementation time, ...) and to increase productivity for the manufacture of cemented parts.
The steel according to the invention thanks to its mastered quenchability allows also to reduce the pressure of quenching gases to obtain a depth of same carburizing, which can further reduce or eliminate deformations on cemented parts and to obtain gains and simplifications on gas tempering technologies of parts in furnace gas quenching furnaces.
We also proceeded with carburizing under low pressure of non-notched resilience specimens (Dimensions: L = 55mm, section 10x1Onnm) at high temperature (980 C), on the one hand on the reference steel A

before gas quenching under a pressure of 20 bar, and secondly on steel E
according to the invention but here before a quenching gas under a pressure of 1.5 bars only. The case hardened depths were identical, and than the nature of the quenching gas. The test pieces thus hardened and tempered 5 were then ruptured by shock at room temperature. The results of rupture energy thus obtained were respectively:
- 19 Joules for reference steel A
- 29 Joules for steel E according to the invention.
In parallel, it has been cemented under low pressure at a temperature standard (930 C) of the test specimens of the reference steel A, for get the same cemented depth as above. They were then soaked with the same gas under a pressure of 20 bar. These specimens were ruptured as above at room temperature and the energy of resulting breakage was 17 Joules, significantly less than for The steel E according to the invention cemented at high temperature.
This shows that despite a hardness in the heart of the specimen of steel A
(312 HV) lower than for steel E according to the invention (500 H V) the tenacity of high-temperature cemented steel E is higher than that reference steel A cemented at high temperature or temperature usual, for the same final cemented depth. In other words, the fact to use a steel according to the invention to carry out a cementation with high temperature, intended to obtain a given cemented depth, not penalize, on the contrary, the tenacity of cemented parts made with this steel with respect to the use of a reference steel, cemented also at high temperature or usual carburizing temperature to obtain the same cemented depth. The hardness difference at heart between the two steels is not penalizing from this point of view. This also shows that steels the invention are particularly suitable for high-temperature cementation temperature, both to reduce carburizing times, increase productivity and reduce the cost of carburizing, compared to known steels case hardened usual temperature or high temperature. Usage properties obtained on parts, such as toughness, are not degraded compared to steels of reference.

We also proceeded under the conditions already specified in the low-temperature cementation at high temperature (980 C) of test pieces of fatigue-bending of steel E according to the invention comprising in their center a flared U-notch. It was followed by a quenching gas pressure of 1.5 Under these conditions, the endurance limit at 10 million cycles of the steel E according to the invention was 1405 MPa, and that of the steel A of 1165 This shows that using a steel according to the invention to perform a carburizing at high temperature, intended to obtain a depth given cementation, does not penalize fatigue-bending behavior, but at contrary to it is very favorable compared to conventional cementation It should be added here that these fatigue-bending tests are intended to simulate the fatigue resistance of a pinion gear tooth, gear or workpiece the properties of use obtained on parts such as fatigue-bending resistance in tooth root of a pinion or cemented gear.

Claims (9)

1. Steel for mechanical parts, characterized in that its composition is, in weight percentages:
- 0.19% <= C <= 0.25%;
- 1.1% <= Mn <= 1.5%;
- 0.8% <= If <= 1.2%;
0.01% <= S <= 0.09%;
- traces <= P <= 0.025%;
- traces <= Ni <= 0.25%;
- 1% <= Cr <= 1.4%;
- 0.10% <= Mo <= 0.25%;
- traces <= Cu <= 0.30%;
- 0.010% <= Al <= 0.045%;
- 0.010% <= Nb <= 0.045%;
0.0130% <= N <= 0.0300%;
Ti <= 0.005%;
the rest being iron and impurities resulting from the elaboration, the composition chemical being adjusted so that the average values J3m, J11m, J15m and J25m of five Jominy essays are such that:
.alpha. = | J11m - J3m X 14/22 - J25m X 8/22 | <= 2.5 HRC; and .beta. = J3m - J15m <= - 9 HRC. 2. Steel for mechanical part according to claim 1, characterized in that than its composition is, in percentages by weight: traces <= Bi <=
0.10% and / or traces <=
Pb <= 0.12% and / or traces <= Te <= 0.015% and / or traces <= Se <= 0.030% and / or traces <=
Ca <= 0.0050%. 3. Steel for mechanical parts according to claim 1 or 2, characterized in what its composition is adjusted so that:

.beta. = J3m - J15m <= 8 HRC. 4. Steel for mechanical parts according to any one of claims 1 at 3, characterized in that its composition is:
- 0.19% <= C <= 0.25%;
1.2% <= Mn <= 1.5%;
- 0.85% <= If <= 1.2%;
0.01% <= S <= 0.09%;
- traces <= P <= 0.025%;
- 0.08% <= Ni <= 0.25%;
- 1.1% <= Cr <= 1.4%;
- 0.10% <= Mo <= 0.25%;
0.06% <= Cu <= 0.30%;
- 0.010% <= Al <= 0.045%;
- 0.015% <= Nb <= 0.045%;
0.0130% <= N <= 0.0300%;
Ti <= 0.005%;
- traces <= Bi <= 0.07% and / or traces <= Pb <= 0.12%
and / or traces <= Te <=
0.010% and / or traces <= Se <= 0.020% and / or traces <= Ca <= 0.0045%, the rest being iron and impurities resulting from the elaboration. 5. Steel for mechanical parts according to claim 4, characterized in that than its composition is:

- 0.20% <= C <= 0.25%;
- 1.21% <= Mn <= 1.45%;
- 0.85% <= If <= 1.10%;
0.01% <= S <= 0.08%;
- traces <= P <= 0.020%;
- 0.08% <= Ni <= 0.20%;
- 1.10% <= Cr <= 1.40%;
- 0.11% <= Mo <= 0.25%;
0.08% <= Cu <= 0.30%;
- 0.010% <= Al <= 0.035%;
- 0.025% <= Nb <= 0.040%;
- 0.0130% <= N <= 0.0220%;
Ti <= 0.005%;
- traces <= Bi <= 0.07% and / or traces <= Pb <= 0.12%
and / or traces <= Te <=.
0.010% and / or traces <= Se <= 0.020% and / or traces <= Ca <= 0.0045%, the rest being iron and impurities resulting from the elaboration. 6. Process for manufacturing a mechanical part made of case-hardened steel or characterized in that for this purpose a steel is used according to one of 1 to 5, on which machining, carburizing or carbonitriding and quenching. 7. Method according to claim 6, characterized in that said cementation or carbonitriding takes place at a temperature of 950 to 1050 ° C. 8. Mechanical steel part, characterized in that it is obtained by the process according to claim 6 or 7. 9. Mechanical part according to claim 8, characterized in that is of a piece of pignonnerie.